The invention is based on a pressure fluid reservoir as defined hereinafter. In a known reservoir of this kind disclosed in U.S. Pat. No. 4,685,491, the cap of the divider piston has a tubular extension that protrudes above the level of the fluid seal. First, this provides communication between the gas chamber located above the piston and the hollow chamber in the piston; second, it is intended to prevent the fluid from draining into the hollow chamber. However, such drainage is not entirely precluded, because if the pressure fluid reservoir, installed in a motor vehicle, for instance, is severely jarred, the fluid can get into the extension and thus flow into the hollow chamber of the piston, and so this portion of the fluid can no longer contribute to lubricating the piston seals or to sealing off the gas chamber.